280 research outputs found
Separated at Birth: Jet Maximization, Axis Minimization, and Stable Cone Finding
Jet finding is a type of optimization problem, where hadrons from a
high-energy collision event are grouped into jets based on a clustering
criterion. As three interesting examples, one can form a jet cluster that (1)
optimizes the overall jet four-vector, (2) optimizes the jet axis, or (3)
aligns the jet axis with the jet four-vector. In this paper, we show that these
three approaches to jet finding, despite being philosophically quite different,
can be regarded as descendants of a mother optimization problem. For the
special case of finding a single cone jet of fixed opening angle, the three
approaches are genuinely identical when defined appropriately, and the result
is a stable cone jet with the largest value of a quantity J. This relationship
is only approximate for cone jets in the rapidity-azimuth plane, as used at the
Large Hadron Collider, though the differences are mild for small radius jets.Comment: 7 pages, 2 tables; v2: references added; v3: small clarifications and
table 2 added to match journal versio
Searching for an invisible A' vector boson with DarkLight
High-luminosity experiments are able to search for new physics at low
energies, which could have evaded detection thus far due to very weak couplings
to the Standard Model. The DarkLight experiment at Jefferson Lab is designed to
search for a new U(1) vector boson A' in the mass range 10-100 MeV through its
decay A' -> e+ e-. In this paper, we demonstrate that DarkLight is also
sensitive to an A' decaying to invisible final states. We analyze the DarkLight
reach for invisible A' bosons assuming a nominal two month running time,
including the possibility of augmenting the DarkLight design to include photon
detection. We also propose two new analysis techniques that might prove useful
for other high-luminosity searches: a cut on missing energy to improve the
invariant mass resolution, and a cut on the sign of the missing invariant
mass-squared to mitigate pileup. We compare the DarkLight reach to existing
experimental proposals, including a complementary search using the VEPP-3
positron beam.Comment: 18 pages, 22 figures, 4 tables; v2: references and clarifications
added; v3: version published in PRD, discussion of constraints from rare kaon
decays adde
Resolving Boosted Jets with XCone
We show how the recently proposed XCone jet algorithm smoothly interpolates
between resolved and boosted kinematics. When using standard jet algorithms to
reconstruct the decays of hadronic resonances like top quarks and Higgs bosons,
one typically needs separate analysis strategies to handle the resolved regime
of well-separated jets and the boosted regime of fat jets with substructure.
XCone, by contrast, is an exclusive cone jet algorithm that always returns a
fixed number of jets, so jet regions remain resolved even when (sub)jets are
overlapping in the boosted regime. In this paper, we perform three LHC case
studies---dijet resonances, Higgs decays to bottom quarks, and all-hadronic top
pairs---that demonstrate the physics applications of XCone over a wide
kinematic range.Comment: 36 pages, 25 figures, 1 table; v2: references added; v3: discussion
added and new appendix B to match JHEP versio
Aspects of Jets at 100 TeV
We present three case studies at a 100 TeV proton collider for how jet
analyses can be improved using new jet (sub)structure techniques. First, we use
the winner-take-all recombination scheme to define a recoil-free jet axis that
is robust against pileup. Second, we show that soft drop declustering is an
effective jet grooming procedure that respects the approximate scale invariance
of QCD. Finally, we highlight a potential standard candle for jet calibration
using the soft-dropped energy loss. This latter observable is remarkably
insensitive to the scale and flavor of the jet, a feature that arises because
it is infrared/collinear unsafe, but Sudakov safe.Comment: 9 pages, double column, 7 figures, based on a talk by A.L. at the
"Workshop on Physics at a 100 TeV Collider" at SLAC from April 23-25, 2014;
v.2: PRD versio
Aspects of Track-Assisted Mass
Track-assisted mass is a proxy for jet mass that only uses direction
information from charged particles, allowing it to be measured at the Large
Hadron Collider with very fine angular resolution. In this paper, we introduce
a generalization of track-assisted mass and analyze its performance in both
parton shower generators and resummed calculations. For the original
track-assisted mass, the track-only mass is rescaled by the charged energy
fraction of the jet. In our generalization, the rescaling factor includes both
per-jet and ensemble-averaged information, facilitating a closer correspondence
to ordinary jet mass. Using the track function formalism in electron-positron
collisions, we calculate the spectrum of generalized track-assisted mass to
next-to-leading-logarithmic order with leading-order matching. These resummed
calculations provide theoretical insight into the close correspondence between
track-assisted mass and ordinary jet mass. With the growing importance of jet
grooming algorithms, we also calculate track-assisted mass on soft-drop groomed
jets.Comment: 35+17 pages, 22 figures; v3: improvements to calculation and
presentation to appear in JHE
Unsafe but Calculable: Ratios of Angularities in Perturbative QCD
Infrared- and collinear-safe (IRC-safe) observables have finite cross
sections to each fixed-order in perturbative QCD. Generically, ratios of
IRC-safe observables are themselves not IRC safe and do not have a valid
fixed-order expansion. Nevertheless, in this paper we present an explicit
method to calculate the cross section for a ratio observable in perturbative
QCD with the help of resummation. We take the IRC-safe jet angularities as an
example and consider the ratio formed from two angularities with different
angular exponents. While the ratio observable is not IRC safe, it is "Sudakov
safe", meaning that the perturbative Sudakov factor exponentially suppresses
the singular region of phase space. At leading logarithmic (LL) order, the
distribution is finite but has a peculiar expansion in the square root of the
strong coupling constant, a consequence of IRC unsafety. The accuracy of the LL
distribution can be further improved with higher-order resummation and
fixed-order matching. Non-perturbative effects can sometimes give rise to order
one changes in the distribution, but at sufficiently high energies Q, Sudakov
safety leads to non-perturbative corrections that scale like a (fractional)
power of 1/Q, as is familiar for IRC-safe observables. We demonstrate that
Monte Carlo parton showers give reliable predictions for the ratio observable,
and we discuss the prospects for computing other ratio observables using our
method.Comment: 41 pages, 14 figures, 1 table, small changes in v.
On the Topic of Jets: Disentangling Quarks and Gluons at Colliders
We introduce jet topics: a framework to identify underlying classes of jets
from collider data. Because of a close mathematical relationship between
distributions of observables in jets and emergent themes in sets of documents,
we can apply recent techniques in "topic modeling" to extract jet topics from
data with minimal or no input from simulation or theory. As a proof of concept
with parton shower samples, we apply jet topics to determine separate quark and
gluon jet distributions for constituent multiplicity. We also determine
separate quark and gluon rapidity spectra from a mixed Z-plus-jet sample. While
jet topics are defined directly from hadron-level multi-differential cross
sections, one can also predict jet topics from first-principles theoretical
calculations, with potential implications for how to define quark and gluon
jets beyond leading-logarithmic accuracy. These investigations suggest that jet
topics will be useful for extracting underlying jet distributions and fractions
in a wide range of contexts at the Large Hadron Collider.Comment: 8 pages, 4 figures, 1 table. v2: Improved discussion to match PRL
versio
Resurrecting the Dead Cone
The dead cone is a well-known effect in gauge theories, where radiation from
a charged particle of mass m and energy E is suppressed within an angular size
of m/E. This effect is universal as it does not depend on the spin of the
particle nor on the nature of the gauge interaction. It is challenging to
directly measure the dead cone at colliders, however, since the region of
suppressed radiation either is too small to be resolved or is filled by the
decay products of the massive particle. In this paper, we propose to use jet
substructure techniques to expose the dead cone effect in the strong-force
radiation pattern around boosted top quarks at the Large Hadron Collider. Our
study shows that with 300/fb of 13-14 TeV collision data, ATLAS and CMS could
obtain the first direct evidence of the dead cone effect and test its basic
features.Comment: 12 pages, 12 figures; v2: references added; v3: approximate version
to appear in PR
Dark Matter, Shared Asymmetries, and Galactic Gamma Ray Signals
We introduce a novel dark matter scenario where the visible sector and the
dark sector share a common asymmetry. The two sectors are connected through an
unstable mediator with baryon number one, allowing the standard model baryon
asymmetry to be shared with dark matter via semi-annihilation. The present-day
abundance of dark matter is then set by thermal freeze-out of this
semi-annihilation process, yielding an asymmetric version of the WIMP miracle
as well as promising signals for indirect detection experiments. As a proof of
concept, we find a viable region of parameter space consistent with the
observed Fermi excess of GeV gamma rays from the galactic center.Comment: 20+12 pages, 11 figures, 1 table; v2: references added, minor
corrections to CMB bounds; v3: footnotes added for clarification, updated
appendix A, conclusions unchanged, version to appear in JCA
Jet Observables Without Jet Algorithms
We introduce a new class of event shapes to characterize the jet-like
structure of an event. Like traditional event shapes, our observables are
infrared/collinear safe and involve a sum over all hadrons in an event, but
like a jet clustering algorithm, they incorporate a jet radius parameter and a
transverse momentum cut. Three of the ubiquitous jet-based observables---jet
multiplicity, summed scalar transverse momentum, and missing transverse
momentum---have event shape counterparts that are closely correlated with their
jet-based cousins. Due to their "local" computational structure, these jet-like
event shapes could potentially be used for trigger-level event selection at the
LHC. Intriguingly, the jet multiplicity event shape typically takes on
non-integer values, highlighting the inherent ambiguity in defining jets. By
inverting jet multiplicity, we show how to characterize the transverse momentum
of the n-th hardest jet without actually finding the constituents of that jet.
Since many physics applications do require knowledge about the jet
constituents, we also build a hybrid event shape that incorporates (local) jet
clustering information. As a straightforward application of our general
technique, we derive an event-shape version of jet trimming, allowing
event-wide jet grooming without explicit jet identification. Finally, we
briefly mention possible applications of our method for jet substructure
studies.Comment: v2 - 31 pages, 18 figures; update to JHEP version, section 3.2
expanded, reference to FastJet contrib updated, results unchange
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